Method and apparatus for making optical fiber preforms and optical fibers fabricated therefrom

ABSTRACT

A method and apparatus are provided for drawing a self-aligned core fiber free of surface contamination and inserting the core fiber into a cladding material to make an optical fiber preform. Single or multi-mode optical fibers having high quality core-clad interfaces can be directly drawn from the preforms described herein.

This is a divisional of application Ser. No. 08/347,978 filed Dec. 1,1994, now U.S. Pat. No. 5,573,571.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to multi-mode and single modeoptical fibers. Specifically, the invention relates to a novel methodfor drawing and inserting a core fiber into a cladding material to forman optical fiber preform having a high quality core-clad interface foruse in fabricating a low lose optical fiber.

2. Description of the Related Art

Single mode optical fibers that transmit both visible and infraredenergy are desirable for use in long distance communications systems.Minimization of transmission loss is particularly important in preparingoptical fibers for use in long distance applications. Conventionalmethods for fabricating optical fibers involve casting glass melts intoglass preforms which are then drawn into optical fibers.

Preforms comprising multicomponent fluoride glasses are typically usedto fabricate low loss optical fibers, particularly, single mode fiberswhich can carry signals over distances of several thousand kilometerswithout the need for repeaters that regenerate the signal.Multicomponent fluoride glasses are particularly susceptible tocrystallite formation since these glasses have significantly lowertheoretical transparencies than silica glasses and exhibit lowviscosities at their liquidus temperatures. Furthermore, the melts ofmulticomponent glasses are reactive with ambient gases and cruciblematerials which increases their susceptibility to crystallization andcontamination.

Conventional methods for making optical fiber preforms expose the coreand cladding materials to temperatures exceeding crystallizationtemperatures during the addition of the core material to the claddingmaterial. These methods involve cooling a cladding glass melt and asubsequent reheating of the cladding glass to temperatures greater thanthe crystallization temperature upon the addition of a core glass meltto the cladding glass. Conventional methods for forming optical fiberpreforms such as, for example, suction casting, build-in-casting,rotational casting and rod-in-tube casting are described in Chapter 5 ofFluoride Glass Fiber Optics, Academic Press, Inc., edited by Ishivar D.Aggarwal and Grant Lu, pp. 223-227 (1991). See also, for example, U.S.Pat. Nos. 4,793,842, 5,106,400 and 5,160,521 the disclosures of whichare incorporated herein by reference.

Preforms cast by conventional methods must be modified before beingdrawn into single mode fibers having desired core and claddingdiameters. Examples of such modifications include stretching proceduresat high temperatures which further increase crystallite formation andmultiple jacketing procedures.

SUMMARY OF THE INVENTION

The subject invention is directed to a method for making an opticalfiber preform utilizing a movable fiber drawing member. The methodcomprises the steps of axially aligning a fiber drawing member and acontainment vessel having a molten core material contained therein, andmoving at least an end portion of the fiber drawing member into thecontainment vessel so as to contact the molten core material. Theviscosity of the core material is then increased to at least about 10⁵poises, and the end portion of the fiber drawing member is removed fromthe containment vessel so as to form a core fiber from the corematerial. Thereafter, the core fiber is cleaved at a predeterminedlocation spaced from the end portion of the fiber drawing member to forma core fiber having a predetermined length. Subsequently, the length ofthe core fiber is aligned with a second containment vessel having amolten cladding material contained therein and the fiber drawing memberis moved toward the second containment vessel so as to introduce thelength of core fiber into an inner portion of the molten claddingmaterial to form an optical fiber preform. Preferably, the methodfurther comprises inserting the core fiber into the cladding materialunder vacuum or inert atmosphere. The core fiber is preferably insertedinto the cladding material when the temperature of the inner portion ofthe cladding material is below the crystallization temperature and abovethe glass transition temperature to provide a preform having a highquality core-clad interface.

A method for making an optical fiber is also provided comprising thesteps of forming the optical fiber preform and drawing the preform intoan optical fiber. Optical fibers including multi-mode or single modefibers having desirable core-clad ratios, may be directly drawn from thepreforms described herein without the need for modifications of thepreforms. In one aspect, single mode heavy metal fluoride optical fibersare directly drawn from the preforms cast in accordance with the methoddescribed herein.

The subject invention is also directed to an apparatus for making anoptical fiber preform which comprises a carriage slidably mounted on asupport structure and configured to translate in a plane of movementwith respect to the support structure. Means are associated with thecarriage for supporting at least a portion of a core fiber, and aplatform is associated with the support structure to receive acontainment vessel. The platform is configured for movement in a planeperpendicular to the plane in which the carriage translates. Theapparatus also includes a mechanism for moving the platform in such amanner so as to axially align a containment vessel and a core fibersupported by the fiber supporting means. Mechanism for heating acontainment vessel and an end portion of the core fiber supporting meansare also provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view in cross-section of an apparatus for forming andinserting a self-aligned core fiber into a cladding material to form apreform in accordance with the present invention wherein an end portionof a fiber drawing member is moved into a first containment vessel tocontact a core material disposed therein.

FIG. 1A is a side view in cross-section of the core fiber drawing andpreform casting apparatus illustrating formation of a core fiber fromcore material in the first containment vessel.

FIG. 1B is a side view in cross-section of the core fiber drawing andpreform casting apparatus illustrating a core fiber attached to an endportion of the fiber drawing member.

FIG. 1C is a side view in cross-section of the core fiber drawing andpreform casting apparatus of the present invention illustratinginsertion of a core fiber into a cladding material in a containmentvessel located on a platform of the apparatus.

FIG. 2 is a cross-sectional view of a second containment vessel on theplatform of the core fiber drawing and preform casting apparatus takenalong lines 2--2 of FIG. 1C illustrating the relationship between theouter and inner portion of the cladding material relative to the corefiber during the formation of the preform.

FIG. 3A is a graphic illustration of typical temperatures of the preformcasting vessel, core material, and cladding material during aconventional preform casting method.

FIG. 3B is a graphic illustration of typical temperatures of thecontainment vessel core fiber and cladding material during the coreinsertion technique of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preforms prepared in accordance with the method described herein includecore and cladding materials. The core and cladding materials arepreferably composed of glasses, particularly heavy metal nonoxideglasses or oxide glasses. Core and cladding glasses described herein areprepared under dry inert conditions from high purity commerciallyavailable reagents. Materials which exhibit a greater viscosity changewith temperature are desirable for use in preparing the preforms sincethey offer a decreased resistance to the inserted core fiber. Glassescomprising silicates, borates, halides, or chalcogenides are usefulmaterials for preparing the preforms. Multicomponent glasses includingone or more glass formers such as silicon oxide or boron oxide and othermetal oxides which are not glass formers can be used to make thepreforms. The present invention is not limited to use of any of theseparticular glasses. Indeed, one skilled in the art can employ variousother materials to prepare the preforms in accordance with the methoddescribed herein. The core insertion method of the present invention isespecially useful for making preforms comprising grossly different coreand cladding materials having dissimilar indices of refraction andthermal expansion coefficients. For example, the core fiber may comprisehalides and the cladding material may comprise silicate glass.

Particular glass compositions can be combined to achieve largernumerical apertures and vary indices of refraction. The composition ofthe cladding glass should preferably have a lower index of refractionthan that of the core glass. Useful dopants for modifying the refractiveindices of the core and cladding glass compositions are LiF, HfF₄, PbF₂,AlF₃ and BiF₃. Halide glasses, particularly heavy metal fluoride glasses(HMF), are preferred materials for preparing optical fibers due to theirlow phonon energy and wide transmission window. HMF glasses are alsodesirable hosts for rare earth doping since they have a high solubilityfor rare earth ions.

HMF glasses have narrow working temperature ranges of about 80° C. toabout 150° C. between crystallization temperatures (T_(x)) and glasstransition temperatures (T_(g)). Preforms comprising HMF glassesproduced by conventional casting methods are particularly susceptible tocrystallite formation upon the exposure of the glasses to temperaturesnear or above crystallization temperatures. Thus, the core insertionmethod described herein is particularly useful for preparing HMF glasspreforms. Preferably, the core and cladding glasses comprisefluorozirconates having zirconium fluoride as the predominant componentas well as modifiers and stabilizers comprising fluorides of barium,lanthanum, sodium, aluminum, lithium, gadolinium and lead. Examples ofsuitable fluorozirconate glasses include zirconium-barium-gadoliniumfluoride (ZBG), zirconium-barium-gadolinium-aluminum fluoride (ZBGA) andzirconium-barium-lanthanum-aluminum fluoride (ZBLA). A preferred glassfor use in fabricating the preforms is ZBLAN glass which is formed bythe addition of sodium fluoride to further stabilize ZBLA glass. Themost preferred cladding glass for use with this invention is HBLAN glasswherein hafnium tetra fluoride (HfF₄) is substituted for zirconiumfluoride (ZrF₄) to reduce the index of refraction of the ZBLAN glass.The most preferred core glass is ZBLAN glass doped with up to 10% PbF₂to increase the index of refraction. Other useful dopants for modifyingthe refractive indices of the fluoride glass compositions are LiF, AlF₃and BiF₃.

The method described herein for forming a core fiber and inserting thecore fiber into a cladding material is preferably performed in a glovebox having a controlled atmosphere.

With reference to FIG. 1, an apparatus, designated generally byreference numeral 10, in accordance with one embodiment of the presentinvention includes a platform 11 associated with a support structure 12wherein platform 11 is configured for receiving a containment vessel. Afiber drawing member 14 is associated with a carriage 16 slidablymounted on support structure 12, wherein support structure 12 defines anaxis generally parallel thereto designated by reference "L". Carriage 16is adapted to reciprocally translate along the aforementioned "L" axisas defined by support structure 12. Further, platform 11 is configuredto reciprocally translate in a plane perpendicular to the plane in whichthe carriage 16 translates so as to effect axial alignment of atemperature controlled end portion 18 of fiber drawing member 14 with aconical end portion 21 of a cylinder 22. The cylinder 22 is preferablydisposed in the central portion of a first containment vessel 20 uponplatform 11. Preferably, the fiber drawing member 14 has a tapered endportion, wherein the tapered end portion 18 includes platinum, gold, orcarbon. However, it is appreciated that the tapered end portion 18 isnot to be limited to platinum, gold, or carbon, but rather may encompassany substitutable material, i.e., a material that does not react withthe molten glass. In the present preferred embodiment, the fiber drawingmember 14 is preferably configured to have a removable portion 15including the end portion 18.

The apparatus 10 further includes a first actuating mechanism 24configured for effecting movement of platform 11 in a first lineardirection and a second actuating mechanism 26 configured for effectingmovement of platform 11 in a second linear direction, wherein the secondlinear direction is generally perpendicular to the aforementioned firstlinear direction. However, it is to be appreciated that platform 11 isnot to be limited to the foregoing first and second linear directionsbut rather may be configured for movement in any direction along a planegenerally perpendicular to the plane in which carriage 16 translates.The apparatus 10 further includes a heating mechanism 28 operativelyassociated with the fiber drawing member 14, whereby the heatingmechanism 28 is configured to maintain the temperature of the fiberdrawing member end portion 18 at a constant temperature somewhat lessthan the glass softening temperature of the core material to preventdamage to the interface between the core fiber and the end portion 18,by thermal expansion. The end portion 18 of the fiber drawing member 14is preferably maintained at a temperature of from about 230° C. to about260° C. Preferably, the diameter of end portion 18 is about 0.5 mm toabout 6 min. A diameter of from about 1.0 mm to about 1.5 mm ispreferred, and a diameter of 1 mm is most preferred.

Core material prepared from high purity commercially available reagentsin a controlled atmosphere glove box is melted, preferably, in a vitrouscarbon or platinum crucible under a dry argon or reactivesulfur-hexafluoride atmosphere at temperatures of from about 700° C. toabout 1000° C. A core material melted in a vitrous carbon crucible canthen be exposed to an oxygen atmosphere to remove carbon particles fromthe material.

With continued reference to FIG. 1, molten core material 30 isintroduced into a chamber 31 as defined by the first containment vessel20. The first vessel 20 is preferably a cylindrical brass quencherwherein the chamber 31 includes a metallic coating. A heating mechanism32 is associated with the first vessel 20 to maintain the temperature offirst vessel 20 at a constant temperature somewhat less than the glasstransition temperature. The temperature of first vessel 20 is preferablyless than about 260° C. A vessel temperature of about 200° C. is mostpreferred for providing a core material having a desired viscosity fordrawing a clear and undamaged core fiber. The core material 30solidifies from the outer portion of the core material adjacent theperipheral wall of the first containment vessel 20 toward the innerportion of the core material. The core material constricts as itsolidifies forming a crest 25 (FIG. 1) over the conical end portion ofcylinder 22 in first vessel 20. It is to be appreciated that suchheating mechanism 32 is well known in the art and need not be describedhereinafter.

Referring now to FIGS. 1A-1C, in conjunction with FIG. 1, the operationof apparatus 10 will now be described. The end portion 18 of fiberdrawing member 14 is introduced into the first vessel 20 to contact thecrest 25 of the solidifying core glass 30, as shown in FIG. 1. Referringto FIG. 1A, carriage 16 is approximated away from the first vessel 20 soas to form a core fiber 34 from the first vessel 20. A first end of corefiber 34 remains attached to end portion 18 after core fiber 34 is drawnfrom the core material 30. The carriage 16 is preferably translated whenthe viscosity of the core material 30 reaches about 10⁶ poises.

It is to be appreciated that a core fiber drawn to a diameter of about50 to about 2000 microns is preferred for use in making a preform to bedrawn into a single mode fiber. The diameter of a core fiber isdependent on the viscosity of the core material and the speed at whichthe core fiber is drawn. Core fibers having smaller diameters can beobtained by increasing the draw speed. Conversely, core fibers havinglarger diameters can be obtained by decreasing the draw speed.

Core fiber 34 is preferably drawn from core material 30 approximately 1minute after pouring the core material 30 into the first vessel 20. Thecore fiber 34 is formed at a draw speed of from about 1.25 centimetersto about 5 centimeters per second. Preferably, the core fiber 34 isdrawn to a length of about 0.5 centimeters to about 50 centimeters.

The temperature of core material 30 is preferably above the glasstransition temperature and below the crystallization temperature of thecore material at the time of drawing the core fiber 34. It is noted,that for a core material comprising ZBLAN glass, the core fiber ispreferably drawn when the glass has a temperature of about 310° C. toobtain a core fiber having a diameter of about less than 2 min. A corefiber having a larger diameter of about greater than 2 mm can beobtained when the core fiber is drawn from a core material having atemperature of about 300° C. or less and a viscosity of about greaterthan 10⁷ poises.

Referring to FIG. 1B, core fiber 34 is cleaved at a predeterminedlocation spaced from the end portion of the core fiber to a length offrom about 5 mm to about 200 mm. The core fiber 34 remains attached tothe end portion 18 of fiber drawing member 14.

The core fiber can have a circular or non-circular cross-sectionalgeometry. Examples of non-circular shaped core fibers include square,triangular, elliptical and helical core fibers. The shape of a corefiber 34 is determined by the shape of the end portion 18 of the fiberdrawing member 14. The core fibers described herein are free of surfacehydroxide compounds which typically form on fibers when they are exposedto parts per million levels of moisture for extended periods, since thecore fibers are drawn and not handled prior to directly inserting thecore fiber into a cladding material.

With continued reference to FIG. 1B, the first vessel 20 is removed fromplatform 11 and a second containment vessel 36 is placed on platform 11.Platform 11 is then moved to align the core fiber 34 with the centralportion of second vessel 36.

A cladding material 38 prepared from high purity commercially availablereagents in a controlled atmosphere glove box is melted, preferably, ina platinum or vitrous carbon crucible at a temperature of about 800° C.in an SF₆ atmosphere. The cladding material can then further be exposedto a dilute oxygen atmosphere to remove carbon particles.

Molten cladding material 38 cooled to about 600° to about 700° C. isintroduced into a preheated temperature controlled second vessel 36 toform an inner cladding portion and an outer cladding portion in thevessel. Second vessel 36 is preferably a mold having a cylindricalchamber and a peripheral wall. The cylindrical chamber of second vessel36 also preferably includes a metallic coating. A heating mechanism 40associated with second vessel 36 is employed to maintain the temperatureof the vessel at a constant temperature somewhat less than the glasstransition temperature (T_(g)) during the core insertion procedure. Thetemperature of second vessel 36 is preferably maintained at about 260°C.

Referring now to FIG. 1C, carriage 16 is translated along theaforementioned "L" axis as defined by support structure 12 to rapidlyinsert the core fiber 34 into the inner portion of the cladding material38 immediately before the inner cladding material completely solidifies.The present invention provides a method for precise axial insertion ofthe core fiber 34 into the cladding material 38. The core fibersdescribed herein are self-aligned, i.e., the core fibers are drawn fromthe core material 30 and directly inserted into the cladding material 38along the same aforementioned "L" axis thereby avoiding damage to thecore fiber and ensuring successful insertion of the core fiber into thecladding material.

The temperature of the inner cladding portion at the time of insertionof the core fiber is below the crystallization temperature (T_(x)) andabove the glass transition temperature (T_(g)). With reference to FIG.2, once the cladding material 38 is introduced into the second vessel36, solidification of the cladding material occurs from the outerportion of the cladding material 42 adjacent to the inner wall 44 of thesecond vessel 36 toward the inner portion of the cladding material 46into which the core fiber 34 is inserted. At the time of insertion ofthe core fiber, the outer cladding portion 42 is substantiallysolidified while the inner cladding portion 46 remains in a somewhatmolten state. The core insertion should be performed rapidly so that thecore fiber does not soften or dissolve during the procedure.

Upon insertion, the temperature of the core fiber increases to somewhatabove the glass transition temperature and is then rapidly quenchedavoiding bulk crystallization problems. Neither the core fiber norcladding material are exposed to crystallization temperatures uponinsertion of the core fiber into the cladding material. The core fiberis preferably inserted into the cladding material at the lowest possibletemperature before the inner cladding portion completely solidifies sothat the core fiber is not subjected to an undue amount of thermalstress. Since the temperature of the cladding glass decreases rapidlyonce it is introduced into second vessel 36, for a preform having adiameter of about 14 millimeters the core insertion time (t_(crr)) ispreferably from about 80 to about 100 seconds after introducing thecladding glass into second vessel 36. The core insertion time is greaterfor making preforms having larger diameters and shorter for makingpreforms having smaller diameters.

Acceptable temperatures for the core fiber at the time of insertion aretemperatures below the glass transition temperature. The temperature ofthe core fiber at the time of insertion is preferably about roomtemperature. The temperature of the inner cladding portion at the timeof insertion of the core fiber is below the crystallization temperatureand above the glass transition temperature of the cladding material. Thetemperature of a fluoride glass cladding material at the time ofintroducing it into the mold is preferably about 600° C. to about 700°C. For a fluoride glass cladding material, the inner cladding portion ispreferably about 15° C. to about 35° C. below the crystallizationtemperature at the time of insertion of the core fiber. For example, acore fiber should be inserted into a cladding material composed of ZBLANglass having a crystallization temperature of about 355° C., when thetemperature of the inner cladding portion is about 310° C. to about 340°C.

FIG. 3A graphically illustrates typical temperatures of the preformcasting vessel, core material and cladding material during aconventional preform casting method of the prior art. In contrast, FIG.3B graphically illustrates typical temperatures of the containmentvessel, core fiber, and cladding material during a core insertiontechnique of the present invention. The time at which the core melt isintroduced into a cladding melt in the casting vessel of theconventional method is represented as t₁. The time at which the corefiber is inserted into the cladding material in accordance with thepresent invention is represented as t_(crr).

The apparatus depicted in FIGS. 1-1C is not intended to limit the typeof casting apparatus for use in accordance with the present invention toany particular embodiment. One skilled in the art can envision variousmodifications to the apparatus for performing the core insertiontechnique described herein. In one embodiment, the apparatus includesmeans for automatically translating the carriage along a planeperpendicular to the plane in which the platform translates. In anotherembodiment, the apparatus includes a means associated with thecontainment vessel for detecting, controlling and displaying thetemperature of the cladding material in the containment vessel. Theapparatus can also include means for automatically inserting said corefiber into said cladding material when the cladding material reaches adesired temperature. In still another embodiment, the apparatus caninclude a timing means for inserting the core fiber into the claddingmaterial at a predetermined time. Furthermore, a shutter fixed to thefirst end of the second vessel 36 can be employed to prevent surfacecooling and contamination of the cladding material.

The containment vessel of the casting apparatus can have a circular ornon-circular cross-sectional geometry for providing a preform includinga cladding material having a circular or non-circular cross-sectionalgeometry. Furthermore, a preform obtained in accordance with the presentinvention can be introduced into a second cladding material inaccordance with the method described herein. The cladding material ofthe preform can be reduced in size and/or altered in shape prior toinsertion into the second cladding material. The resulting preformhaving two cladding layers can then be inserted into a third claddingmaterial. Thus, preforms having multiple cladding layers can be obtainedin accordance with the method described herein. For example, a preformprepared in accordance with the present invention can include a squareinner cladding material surrounding a core fiber and an outer circularcladding material surrounding the inner cladding material.

After the preform is removed from the casting apparatus, it can then bedirectly drawn into an optical fiber having the desired core andcladding diameters without the need for modification of the preform byadditional stretching and jacketing procedures. The optical fiber can bea single mode or multi-mode fiber. Fiber drawing methods are describedin Fluoride Glass Optical Fibers, P. W. France et al., Blackie CRC PressInc. pp. 114-116 (1990); Encyclopedia of Chemical Technology, John Wiley& Sons, Vol. 10, pp. 131-133 (1980) and Fluoride Glass Fiber Optics, I.D. Aggarwal and G. Lu, Academic Press, Inc. pp. 227-228 (1991) which areincorporated herein by reference.

Single mode fibers drawn from the preforms described herein havediameters of about 100 to about 200 microns and have core diameters ofabout less than 6 microns. Thus, the core diameter of a single modeoptical fiber drawn from a preform prepared in accordance with themethod described herein comprises about less than 3% to 6% of thediameter of the single mode fiber. The core to clad ratio of a preformprepared in accordance with the present invention is equal to the coreclad ratio of an optical fiber that is drawn from that preform. Thus,the necessary diameter of the core fiber to be inserted into thecladding material disposed in a containment vessel having apredetermined diameter is determined by the desired core to clad ratioof the optical fiber to be drawn from the preform. Core to clad ratiosof about 0.005 to about 0.05 are preferred for single mode opticalfibers.

For single mode propagation fibers normalized frequency V is less thanor equal to 2.401. The normalized frequency parameter describes therelationship of the wavelength of the guided light propagating throughthe optical fiber core to the refractive indices of the core and thefiber cladding, and is expressed by the formula

    V=(2πα/λ)(n.sub.core.sup.2 -n.sub.clad.sup.2).sup.1/2

wherein α is the radius of the core of the fiber, λ is the wavelength ofoperation and n is the index of refraction. The numerical aperture ofthe optical fiber is expressed by the formula

    NA=(n.sub.core.sup.2 -n.sub.clad.sup.2).sup.1/2

Therefore, the necessary diameter of the core of the single mode opticalfiber 2α can be determined by the formula 2a<(V×λ)/(2π×NA). A preferredwavelength of operation for a single mode optical fiber is 1.3 μm. Thediameter of the core of a single mode fluoride fiber having a wavelengthof operation of 1.3 μm must be less than 6 microns. Smaller diametercore fibers and larger diameter cladding molds can be employed to makepreforms for fabricating optical fibers having greater numericalapertures and/or shorter wavelengths of operation.

The optical fibers drawn from the preforms of the present invention canbe examined under an interference microscope to inspect the core-cladinterfaces of the optical fibers. Interference microscopy reveals thatoptical fibers drawn from preforms prepared in accordance with the coreinsertion method of the present invention have high quality core-cladinterfaces that are free of crystallites.

The following examples are illustrative of the core insertion method,preforms, and optical fibers of the present invention.

EXAMPLE 1 Preparation of Core Glass and Core Fiber

ZBLAN core glass composed of ZrF₄ (53 mole %), BaF₂ (20 mole %), LaF (4mole %) and NaF (20 mole %) doped with % PbF₂ to increase the index ofrefraction was prepared from high purity commercially availablematerials in a glove box under argon atmosphere. The core glass wasmelted in a vitrous carbon crucible at 800° C. in a 10 kW RF furnaceunder a sulfur hexafluoride atmosphere. The core glass was thentransferred to a platinum crucible and exposed to a dilute oxygenatmosphere at 800° C. to remove carbon particles. The remaining steps ofthe formation of the core fiber described hereinafter were performed ina glove box. The molten glass was cooled to 600° C. before pouring itinto a gold coated brass quencher having a temperature of 200° C. Thequencher included a cylinder having a conical end portion disposed in acentral portion of the quencher. A platinum tapered end portion having atemperature of 250° C. was mounted on a fiber drawing member attached toa carriage of the core fiber drawing and casting apparatus. The platinumtapered end portion was aligned with the conical end portion of acylinder in the quencher by moving a platform of the apparatus in aplane perpendicular to the plane in which the carriage translates.

The platinum tapered end portion was immersed in the core material inthe quencher when the temperature of the inner portion of the corematerial was 310° and the core material reached a viscosity of 10⁶poises. The carriage of the apparatus was raised at a draw speed of 2.5centimeters per second for 10 seconds. The core fiber was drawn to alength of 25 centimeters, and later cleaved to a length of about 10centimeters.

EXAMPLE 2 Preparation of the Cladding Glass

HBLAN glass composed of HfF₄ (53 mole %), BaF₂ (20 mole %), LaF (4 mole%), AlF₃ (3 mole %), and NaF (20 mole %) was prepared from high puritycommercially available reagents in a glove box under argon atmosphere.The cladding glass was melted in a vitrous carbon crucible at 800° C. ina glove box having an SF₆ atmosphere. The molten cladding glass wascooled to 600° C. and then poured into a preheated (250° C.) gold coatedbrass containment vessel in the glove box. The containment vessel had adiameter of 14 mm.

EXAMPLE 3 Insertion of the Core Fiber to Obtain the Preform

The core fiber of Example 1 having a temperature of 25° C. was rapidlyinserted into the cladding glass of Example 2 100 seconds after thecladding glass was poured into the preform casting containment vessel.The temperature of the inner cladding layer at the time of insertion ofthe core fiber was 320° C. The temperature of the mold was held at 260°C. during the insertion procedure. After insertion of the core fiber,the temperature of the preform decreased to room temperature and wasthen removed from the casting apparatus.

The preform was drawn into a single mode fiber and inspected under aninterference microscope. The preform exhibited annular interferencefringes indicating that the core-clad interface was free ofcrystallites.

Other modifications and variations of the present invention are possiblein light of the above teachings. It is therefore, to be understood thatchanges may be made in particular embodiments of the invention describedwhich are within the full intended scope of the invention as defined bythe claims.

What is claimed is:
 1. An optical fiber having a core-clad interfacesubstantially devoid of crystallites drawn from an optical fiber preformobtained utilizing a movable fiber drawing member and comprising thesteps of:a) axially aligning the fiber drawing member and a firstcontainment vessel having a molten core material contained therein; b)moving at least an end portion of the fiber drawing member into thefirst containment vessel so as to contact the molten core material; c)increasing the viscosity of the core material to at least 10⁵ poises; d)removing the end portion of the fiber drawing member from the firstcontainment vessel so as to form a core fiber from the core material; e)cleaving the core fiber at a predetermined location spaced from the endportion of the fiber drawing member to form a core fiber having apredetermined length; f) aligning the predetermined length of the corefiber and a second containment vessel having a molten cladding materialcontained therein; and g) moving the fiber drawing member toward thesecond containment vessel so as to introduce the predetermined length ofcore fiber into an inner peripheral portion of a molten claddingmaterial to form an optical fiber preform.
 2. An optical fiber accordingto claim 1, wherein said optical fiber comprises a single mode opticalfiber.
 3. An optical fiber according to claim 2, wherein the diameter ofthe core of the optical fiber is less than about 6 microns.
 4. Anoptical fiber according to claim 1, wherein the core of the opticalfiber is comprised of heavy metal fluoride glass and the claddingmaterial of the optical fiber is comprised of heavy metal fluorideglass.
 5. An optical fiber according to claim 4, wherein the heavy metalfluoride glass of each of the core and cladding materials is amulticomponent glass selected from the group consisting of ZBL, ZBA,ZBLA, ZBGA, ZBLAN, HBLA and HBLAN.
 6. An optical fiber according toclaim 1, wherein the core fiber and the cladding material compriseglasses selected from the group consisting of silicates, borates,chalcogenides and halides.
 7. An optical fiber according to claim 1,wherein the step of removing the end portion of the fiber drawing memberfrom the first containment vessel so as to form a core fiber to obtainthe optical fiber preform is performed when the core material has aviscosity of from about 10⁵ poises to about 10⁷ poises.
 8. An opticalfiber according to claim 1, wherein the step of removing the end portionof the fiber drawing member from the first containment vessel so as toform a core fiber to obtain the optical fiber preform is performed whenthe core material has a temperature of from about 300° C. to about 330°C.
 9. An optical fiber according to claim 1, wherein the step ofremoving the end portion of the fiber drawing member from the firstcontainment vessel so as to form a core fiber to obtain the opticalfiber preform is performed when the core material has a temperature ofabout 310° C. and a viscosity value of 10⁶ poises.
 10. An optical fiberaccording to claim 1, wherein the step of removing the end portion ofthe fiber drawing member from the first containment vessel so as to forma core fiber to obtain the optical fiber preform is performed by drawingthe end portion at a speed of about 0.1 centimeters per second to about5 centimeters per second.
 11. An optical fiber according to claim 1,wherein the step of removing the end portion of the fiber drawing memberfrom the first containment vessel so as to form a core fiber to obtainthe optical fiber preform is performed by drawing the end portion at aspeed of about 2.5 centimeters per second.
 12. An optical fiberaccording to claim 1, wherein the step of moving at least an end portionof the fiber drawing member into the first containment vessel so as tocontact the molten core material to obtain the fiber optic preform isperformed with the temperature of the end portion at about less than thesoftening temperature of the core material.
 13. An optical fiberaccording to claim 1, wherein the step of moving at least an end portionof the fiber drawing member into the first containment vessel so as tocontact the molten core material to obtain the fiber optic preform isperformed with the temperature of the end portion at from about 230° C.to about 260° C.
 14. An optical fiber according to claim 1, wherein thestep of moving the fiber drawing member toward the second containmentvessel so as to introduce the predetermined length of core fiber into aninner peripheral portion of a molten cladding material to form anoptical fiber preform is performed when the temperature of the claddingmaterial is below its crystallization temperature and above its glasstransition temperature.
 15. An optical fiber according to claim 1,wherein the step of moving the fiber drawing member toward the secondcontainment vessel so as to introduce the predetermined length of corefiber into an inner peripheral portion of a molten cladding material toform an optical fiber preform is performed when the temperature of thecladding material is at about its solidification temperature.